Device for the electrostatic application of protective coatings with synthetic powders by the use of spray guns

ABSTRACT

A material coating system to apply charged materials to objects to be coated by electrostatic spray guns. The spray gun itself has a double cone-shaped deflecting member in its barrel and an adjustable collar on its barrel to change the spray pattern. An internal electrode in the material conveying tube charges the material and a wound wire ground connection attached to the gun prevents an accumulation of charge at the gun. A material storage bin with multiple venturis feeds material to the gun by suction via its conveying tube. In addition, a polarity reversing circuit allows the power source to apply different charges to sprayed materials.

United States Patent Szasz 51 Oct. 17, 1972 s41 DEVICE FOR THEELECTROSTATIC 3,279,429 10/1966 Felici et al. ..239/ 15 X APPLICATION OFPROTECTIVE 3,521,815 7/1970 Szasz ..239/15 COATINGS WITH SYNTHETIC3,575,344 4/1971 Angelico ..239/15 3,608,823 9/1971 Buschor ..239/15POWDERS BY THE USE OF SPRAY GUNS J Inventor: Imre Szasz, St. Gallen,Switzerland Assignee: Graco Inc., Minneapolis, Minn.

Filed: April 23, 1971 Appl. No.: 136,900

Foreign Application Priority Data May 6, 1970 Germany ..P 20 22 088.6

US. Cl ..239/15, 239/518 Int. Cl. ..B05b 5/00 Field of Search ..239/15,3, 518, 515

References Cited UNITED STATES PATENTS Umbricht ..239/518 Hunter..239/515 X Primary Examiner-M. Henson Wood, Jr. AssistantExaminerThomas C. Culp, Jr. Attorney-Thomas Zack [57] ABSTRACT at thegun. A material storage bin with multiple venturis feeds material to thegun by suction via its conveying tube. In addition, a polarity reversingcircuit allows the power source to apply different charges to sprayedmaterials.

10 Claims, 7 Drawing Figures PATENTEDHBI I! 1972 JNVENTOR [mre Szasz ATTORNEY DEVICE FOR THE ELECTROSTATIC APPLICATION OF PROTECTIVE COATINGSWITH SYNTHETIC POWDERS BY THE USE OF SPRAY GUNS The invention involves adevice for the electrostatic application of protective or decorativecoatings with synthetic powders by the use of spray guns. It is known tobe desirable to apply a synthetic coating to the surfaces of those rawmaterials that are not durable or long-lasting; for example, to protectthem against corrosion, abrasion, and to provide electrical insulation.In electrostatic powder coating, the initial electrically neutral(uncharged) character of the powder particles is changed in that theparticles receive an electrostatic charge.

While these particles are being sprayed from the gun, which also usuallyhouses the ionizing element, the particles are being attracted to thegrounded object (object being sprayed) because of their charge finallyreaching the object.

The particles are deposited on the object and are subsequently bakedout" or cured to produce a protective coating.

The electrostatic charge can be applied in the following ways:

1. Through polarization, in which most electrical charge centers in themolecules are displaced or dislocated producing an electrical dipole.

2. Through ionization in which at least one electron is removed or addedto the synthetic material particle.

While the polarization usually occurs through friction of the powder-airmixture on the walls of the conveyance tubes, but can also occur throughfriction between the powder particles themselves; the ionization of theparticles is caused either by the particles coming in contact with ahigh voltage metal electrode or by passing through a high voltage field.Upon being ionized, the particles acquire the polarity of charge, thusthe newly arriving particles are repelled by those already deposited onthe object. With polarization, however, the electrical dipole is alignedin the direction of the grounded object, so that the newly arrivingpowder is attracted both by the grounded objects and by the particlesalready deposited on the object.

Electrostatic powder coating developed from electrostatic paintspraying, in which the high voltage electrode, which effects theionization, is in the muzzle of the spray gun. Since with fluids thereis, practically speaking, no friction, no value was placed onpolarization charging. It could be demonstrated, however, that for theforming of industrially useful coatings, both charging methods aresimultaneously essential.

The essential task of the invention then is to have a device whichallows both of the charging methods to be realized simultaneously. Theinvention accomplishes this so: the flexible insulating hose carrying apowderair mixture, which leads to the muzzle of the gun where there is afixed-position body of revolution, has inside it a thin metal wire whichacts as a high voltage electrode to form a strong corona effect, andthis wire is connected to a high voltage generator, while the outersurface of the cable has a spiraling ground wire around it to introducea high voltage gradient. The separation of the high voltage electrodefrom the head of the spray gun, the safety distance, is important. Atthe muzzle of the gun, a cone-shaped open body of revolution is providedfor the radial deflection of the exiting powder-air mixture.

The flexible insulating hose or conduit is directed through the handleof the gun and is housed within the conveyor pipe, which leads to thevicinity of the muzzle, and the conveyor pipe is housed within an outersliding pipe while the end of the neighboring outlet in relation to theinner surface of the cone of the body of revolution forms the countercone. The supply bin has several parallel-mounted venturis connected toits outlet. These parallel-mounted venturis have a common suctionchamber. The outlet of the supply bin incorporates a continuouslyvariable damper.

The supply bin is tilted in the direction of the exit. Several componentgroups consisting of metering dampers, venturi tubes, and spray guns canbe added to the supply bin. The high voltage generator chopper has abridged capacitor and works directly through a cascade circuit which isconnected directly to the high voltage transformer. The capacitor bridgeand the high voltage transformer operate at or near a resonantfrequency.

The polarity of the cascade output can be changed through polarityreversal of the switching element; it can also be changed by reversingthe cascade rectifier or by transposing the cascade rectifier; and alsoby interchanging the unconnected open rectifier leads. For interchangingthe open ends of the cascade rectifier, an adjustable switching bar canbe provided. Finally, the polarity of the cascade output can be changedby sliding an additional, block-mounted rectifier.

An embodiment of the previously described invention is shown in theaccompanying drawings.

FIG. 1 is a schematic representation and partial section of the muzzlepart of the spray gun showing the deflector cone, powder-air mixtureconveyor pipe and the high voltage electrode.

FIG. 2 is a longitudinal cross-sectional view of the muzzle part of thespray gun showing the deflector cone in an extended position so as toproduce a more diffuse powder spray pattern.

FIG. 3 is a vertical cross-sectional view of the supply bin and themultiple venturi.

FIG. 4 is a vertical cross-sectional view of the supply bin shown inFIG. 3 after it has been rotated about the vertical axis.

FIGS. 5, 6 and 7 show the high voltage generator wiring diagrams as wellas the modification required for the changing of polarity.

Referring now specifically to FIG. 1 embodiment, the electrostatic spraygun assembly has a spray gun for spraying powder-air mixtures with thetube 8 containing the muzzle part of the gun connected to the handle 7.The powder air mixture 1 is conveyed through an insulated flexibleconduit or hose 2 to the spray gun, which is constructed from highresistive materials, making the gun shock and spark proof. The insulatedhose 2 houses a thin electrically conductive high voltage metalelectrode 3 which produces a corona discharge within the tube itself.This high voltage electrode 3 extends only to within about 30-40 mm ofthe powder-air exit passage at the front of the gun 5. This 30-40 mmdistance 20 is termed the safety distance.

The relative motion between the selected powder 1 and the large insidesurface of the insulated hose 2 produces the required friction forpolarization, while the high voltage field produced between the wireshaped high voltage electrode 3 and the ground wire 4 which is spirallywound around the insulated hose 2, produces the ionization. Thisspirally shaped arrangement guarantees an even and repeatable highvoltage field through which the powder particles can pass. The charge ofthe individual powder particles is greatly increased over that obtainedby employing devices in which only one of the charging methods is used.

Positioned at the exit of the spray gun is a generally doublecone-shaped body 16 which serves to deflect the emerging powder-airmixture in a radial direction as shown in FIG. 2. The end of theflexible insulated hose 2, which is housed within the gun, is surroundedby two concentrically positioned pipes 8 and 9, which are constructed ofa nonconductive, hard plastic. The inner pipe or barrel 8 of the gun isfirmly attached to the gun handle 7 while the outer pipe or collar 9 canslide along the inner pipe 8. The outer pipe 9 can be positioned byaxial movement over the cone-shaped bell mouthed opening at the muzzleend of the gun so as to make it possible for the powder-air mixture tobe changed variably from a (beamed) narrow ray of powder as shown inFIG. 1 to a rather diffuse cloud of powder as shown in FIG. 2. Hence theemerging powder pattern can be quickly shaped, without interruption ofthe spraying process, to the most desirable pattern for spraying anobject of a given shape.

The collar 9 encircles substantially the entire longitudinal portion ofthe barrel 8 of the gun. By moving the leading edge of the collarforward of the common base of the two conical shaped members that formthe deflector member 16, the coating material is variably deflected in agenerally forward direction. Deflector member 16, it is noted, resemblestwo conical members of different sizes attached at a common base sidewith the larger cone being forward of the smaller cone and having itsapex at the most forward position of the deflector. An internal chamferor groove at the leading edge of the collar 9 acts as an additionalguide to direct material in a forward direction when the collars leadingedge is moved forward to the juncture of the common base of deflector16. When the collars leading edge is rearward of the common baseattachment, coating material is deflected in a radial direction. A lugor other stop members may be placed on the barrel to limit the forwardand rearward movement of collar 9. As for example, lug 19 rigidlyattached to barrel 8 that slides in a closed slot in collar 9 (FIG. 1).

The supply bin 12 is set at an angle, so'shown in FIG. 3, as to insure acontinuous flow of powder to the chamber 17 of the venturi injectorsystem shown in FIG. 4. The powder-air mixture in the suction chamber 17is then forced with great velocity in the direction of the arrows intothe flexible insulated hose 2. Coating material storage bin 12 storesand dispenses material to conduit 2. This main storage bin 12 has anopening at its lowest side, a large fluid inlet (FIG. 4) with smallerventuris 14 connected to this opening, and an outlet conduit axiallylongitudinally aligned with the fluid inlet that has a diameter or crosssection area greater than the areas of all of the venturi tubescombined.

The continuously variable metering gate or valve 18 shown in FIG. 3controls the flow of powder to the suction chamber 17 to be throttled oreven stopped.

The quantity of powder entrained into the suction chamber 17 dependsdirectly upon the air velocity and flowrate emitted from the venturi,and hence the operating line pressure. For practical as well as economicreasons, line pressures should not exceed the ambient pressure by morethan -84 psi. With these pressure drop limitations, the venturi tubeconveyance method will produce maximum powder flow rates of 30 kg/hour.The maximum powder flow rate can be increased, without increasing linepressure, by employing several parallel mounted venturis 14 which aremounted in a single suction chamber 17 as shown in FIG. 4.

Additional metering gates 18 and venturitubes 14 can be affixed to thesame supply bin 12 so as to permit more than one spray gun to beoperated from a single supply bin 12.

The I-IV generator of this invention works according to the well-knownchopper principle," in which low voltage direct current (8-1 0 volts)from the line rectifier is magnetically or mechanically chopped orrapidly switched. This chopped" DC voltage is passed through avoltage-adjusting potentiometer to the primary side of the RVtransformer, while the secondary side of the HV transformer is attachedto a one-stage or multi-stage cascade multiplier 29, 30, 31, 32,consisting of capacitors 23, 24 and rectifier 25, 26. The cascade alsorectifies the produced high voltage. The chopper 21 is capacitor bridged33, with primary transformer winding and capacitor resonant frequency isapproximately equal to the switching frequency of the chopper so thatthe I-IV transformer and the chopper operate at a resonant frequency,since in this manner the required high voltage can be obtained. Betweenthe output point 29 of the cascade and the gun cable 3, a high ohm-valueresistor is located.

There are powders which produce better operation with positive-polarityhigh voltage and others which operate better with negative-polarity highvoltage. Therefore, the devices, according to the invention, have thecapability to use both polarities. While known devices require twoindependent, correspondingly switched I-IV generators, according to thisinvention both polarities can be obtained by using only one I-IVgenerator and one cascade, in which the cascade has reversible polarity,and specifically:

l. by reversing the cascade rectifiers 25, 26 (FIG. 5);

2. by changing the unconnected ends 29, 30 of the cascade rectifier 25,26 by moving a switching bar 34 and contact 35 (FIG. 6);

3. by location change of the cascade rectifier 25, 26 (FIG. 5);

4. by moving 36 the block mounted supplementary rectifier 37 (FIG. 7).

The foregoing specification accordingly describes an embodiment of thepresent invention and would suggest some of the ways of practicing it tothose skilled in the art. Such a description should not be construed aslimiting the invention as only the foregoing claims can do this.

Now, therefore, what I claim is:

1. A spray gun for applying coating material comprising: a gun barrelhaving a material conveying passage therethrough, said passage having anexit at the front end of said barrel; a collar having a leading edgeencircling a portion of said barrel and movable in the longitudinaldirection of said barrel with the leading edge in the forward mostposition; and a material deflector mounted within the front end of saidbarrel, said deflector being shaped to generally resemble two conicalmembers attached at a common base side and the forward most conicalmember having a larger base side at the side of attachment and an apexat its forward most point, whereby coating material is deflected in aradial direction by the deflector when the leading edge of the collar isrearward of the common base attachment and deflected in a generallyforward direction by the collar as its leading edge is moved forward ofthe common base attachment.

2. The spray gun of claim 1 wherein said collar has an internal chamfernear its leading edge to further assist in directing material from thegun barrel.

3. The spray gun of claim 1 wherein said collar extends substantiallythe entire longitudinal length of said barrel and has means for limitingthe rearward and forward movement of said collar.

4. An electrostatic spray gun assembling comprising: an electrostaticspray gun for applying coating material and having an openingtherethrough to convey material into said gun at an entrance and todischarge the material at the front end of said gun, said gun having abody at least a portion of which is electrically conductive; anelongated flexible material conveying electrically insulating conduitconnectable to said spray gun opening at its entrance; a thinelectrically conductive electrode wire mounted within said conduit alongits longitudinal extent, said wire being connectable to a high potentialsource at one end and terminating at its other end within said gun nearthe front end of said opening; and a second conductive wire wound aroundthe external surface of said material conveying conduit and electricallyconnected to ground potential at one end and to the conductive part ofthe gun body at the second end, whereby said second wire acts as aground connection for the conductive part of the gun body.

5. The assembly of claim 4 wherein said gun body has a handle; and saidconductive portion of the gun body being part of said handle.

6. The assembly of claim 4 including a high potential source, saidsource comprising: a transformer, a capacitor-bridged chopper circuitconnected to the primary winding of said transformer, and a cascademultiplier circuit being connected to the second winding of saidtransformer and also being connected to said electrically conductiveelectrode wire.

7. The assembly of claim 6 wherein the resonant frequency of the circuitformed by said transformer primary winding and bridging capacitor isapproximately equal to the operating switching frequency of saidchopper.

8. The assembly of claim 6 wherein said cascade multiplier circuit hasat least two rectifiers, and means for reversing the polarity of theoutput from the cascade multiplier circuit by reversing said tworectifiers.

9. The assembly of claim 8 wherein said means for reversing said tworectiflers comprises a switch to exchange the leads to said rectifiers.

10. The assembly of claim 6 wherein said means for reversing said tworectifiers comprises three rectifiers only two of which are activecircuit elements at any one time

2. The spray gun of claim 1 wherein said collar has an internal chamfernear its leading edge to further assist in directing material from thegun barrel.
 3. The spray gun of claim 1 wherein said collar extendssubstantially the entire longitudinal length of said barrel and hasmeans for limiting the rearward and forward movement of said collar. 4.An electrostatic spray gun assembling comprising: an electrostatic spraygun for applying coating material and having an opening therethrough toconvey material into said gun at an entrance and to discharge thematerial at the froNt end of said gun, said gun having a body at least aportion of which is electrically conductive; an elongated flexiblematerial conveying electrically insulating conduit connectable to saidspray gun opening at its entrance; a thin electrically conductiveelectrode wire mounted within said conduit along its longitudinalextent, said wire being connectable to a high potential source at oneend and terminating at its other end within said gun near the front endof said opening; and a second conductive wire wound around the externalsurface of said material conveying conduit and electrically connected toground potential at one end and to the conductive part of the gun bodyat the second end, whereby said second wire acts as a ground connectionfor the conductive part of the gun body.
 5. The assembly of claim 4wherein said gun body has a handle; and said conductive portion of thegun body being part of said handle.
 6. The assembly of claim 4 includinga high potential source, said source comprising: a transformer, acapacitor-bridged chopper circuit connected to the primary winding ofsaid transformer, and a cascade multiplier circuit being connected tothe second winding of said transformer and also being connected to saidelectrically conductive electrode wire.
 7. The assembly of claim 6wherein the resonant frequency of the circuit formed by said transformerprimary winding and bridging capacitor is approximately equal to theoperating switching frequency of said chopper.
 8. The assembly of claim6 wherein said cascade multiplier circuit has at least two rectifiers,and means for reversing the polarity of the output from the cascademultiplier circuit by reversing said two rectifiers.
 9. The assembly ofclaim 8 wherein said means for reversing said two rectifiers comprises aswitch to exchange the leads to said rectifiers.
 10. The assembly ofclaim 6 wherein said means for reversing said two rectifiers comprisesthree rectifiers only two of which are active circuit elements at anyone time.